Third-generation cellular phones have been introduced and it is getting required that such phones have not only a speech function but also a high-speed data communications function that enables transmission of electronic mails and image data. It is further required that such phones have a position detecting function.
For example, the law requiring that cellular phones have a position detecting function has been enacted in the United States. In the States, for example, this requirement is accomplished in such a manner that cellular phones for the code division multiple access (that may he hereinafter referred to as CDMA) system are equipped with the receiving function of the global positioning system (GPS) using a frequency band around 1500 MHz as the position detecting function, wherein the phones for the CDMA system are operable in two frequency bands (dual bands) including a frequency band approximately from 800 to 900 MHz that is used in the advanced mobile phone system (AMPS) and a frequency band approximately from 1800 to 2000 MHz that is used in the personal communications service (PCS). It is required that this type of cellular phones deal with these three frequency bands.
If a new function is added to cellular phones as described above, the circuit is made more complicated and the number of components is increased. Therefore, higher-density mounting techniques are required for the cellular phones. Under such a circumstance, it is necessary to achieve a reduction in size and weight of the components and to achieve combination and integration of the components for the high frequency circuits inside the cellular phones to reduce the mounting space.
The Published Unexamined Japanese Patent Application 2003-8385 discloses a composite LC filter circuit for separating the frequency bands of AMPS, PCS and GPS from one another.
The Published Unexamined Japanese Patent Application 2002-101005 discloses a front end module for processing transmission signals and reception signals of each of three communications systems using three frequency bands. This front end module separates a low frequency band from a high frequency band through the use of a diplexer. The low frequency band includes the frequency band of the first communications system. The high frequency band includes the two frequency bands of the second and third communications systems. An antenna switch separates the transmission signals and the reception signals of the first communications system from each other. Another antenna switch separates the reception signals of the second and third communications systems from the transmission signals of the second and third communications systems. Two surface acoustic wave (SAW) filters separate the reception signals of the second communications system from the reception signals of the third communications system. The Published Unexamined Japanese Patent Application 2002-101005 discloses that the components of the front end module are integrated on a multilayer ceramic substrate.
The Published Unexamined Japanese Patent Application 2002-43977 discloses a high frequency module for processing transmission signals and reception signals of each of three communications systems using three frequency bands. This high frequency module separates a low frequency band from a high frequency band through the use of a diplexer. The high frequency band includes the two frequency bands of the first and second communications systems. The low frequency band includes the frequency band of the third communications system. A first high frequency switch separates the reception signals of the first and second communications systems from the transmission signals of the first and second communications systems. A second high frequency switch separates the transmission signals and the reception signals of the third communications system from each other. Two SAW filters separate the reception signals of the first communications system from the reception signals of the second communications system. In addition, the Published Unexamined Japanese Patent Application 2002-43977 discloses that the components of the high frequency module are combined through the use of a layered structure made up of a plurality of stacked sheet layers.
To achieve the above-mentioned cellular phones having the speech function, the high-speed data communications function and the position detecting function, it is desirable to implement front end modules capable of processing transmission signals and reception signals in each of the first and second frequency bands and reception signals for the position detecting function in the third frequency band.
The composite LC filter circuit disclosed in the Published Unexamined Japanese Patent Application 2003-8385 has the function of separating the three frequency bands from one another but does not have any function of separating transmission signals and reception signals from each other in each of the frequency bands. Therefore, when the composite LC filter circuit is used in a communications device such as a cellular phone, it is required to provide a circuit for separating the transmission signals and the reception signals from each other in each of the frequency bands, in addition to the composite LC filter circuit. No consideration is given in the Published Unexamined Japanese Patent Application 2003-8385 to a reduction in size and weight and combination and integration of components in such a case.
The front end module disclosed in the Published Unexamined Japanese Patent Application 2002-101005 separates transmission signals from reception signals through the use of the antenna switches. In the CDMA system, it is required that each of the transmission function and the reception function operates all the time. Consequently, this front end module has a problem that it is not operable in the CDMA system.
The high frequency module disclosed in the Published Unexamined Japanese Patent Application 2002-43977 separates transmission signals from reception signals through the use of the high frequency switch. Therefore, like the front end module of the Published Unexamined Japanese Patent Application 2002-101005, this high frequency module has a problem that it is not operable in the CDMA system.
In the Published Unexamined Japanese Patent Application 2002-101005 and the Published Unexamined Japanese Patent Application 2002-43977, the duplexers including two SAW filters for separating the reception signals of two communications systems from each other are called SAW duplexers. However, duplexers generally mean those for separating transmission signals and reception signals from each other. In the embodiment of the present invention, too, those for separating transmission signals and reception signals from each other are called duplexers. Therefore, the SAW duplexers disclosed in the Published Unexamined Japanese Patent Application 2002-101005 and the Published Unexamined Japanese Patent Application 2002-43977 are different from the duplexers of the embodiment of the present invention, in terms of function.